Patients with hypereosinophilic syndrome 18 years of age and older who have not responded to standard therapy with steroids, interferon and hydroxyurea or cannot take these drugs may be eligible for this study. Candidates will be screened with a medical history, physical examination, eye examination, and blood tests. Depending on the patient's symptoms, other diagnostic tests may be done, including studies of the eyes, lungs, skin, nerves or heart. Skin biopsy and bronchoalveolar lavage may be included in the diagnostic evaluation. For the skin biopsy, a small area about 1/3 inch in diameter is numbed and a small core of skin is surgically removed for study under the microscope. Bronchoalveolar lavage involves inserting a catheter (flexible tube) into the lungs to instill saline (salt water) and obtain a tissue sample. This test will be done only if clinically necessary.